The new Apple payment system has extraordinary promise. With Apple Pay, you might not need a wallet, and you can leave your credit and debit cards at home. In terms of ease and convenience, payment cards represented a big leap from the era of cash. Apple hopes its system will be a comparable leap from the era of cards.

Skeptics have focused on questions of security and privacy, but prospective users might want to pause over a different problem: When payment becomes easier, and when people don't see the money they're handing over, they tend to spend a lot more. And as payment becomes more automatic, people become less sensitive to what they're losing. Apple Pay users might find that their thinner phones are making their bank accounts thinner as well.

A little social science: People who use credit cards tend to give bigger tips at restaurants and spend more at department stores. They are also more likely to forget, or to underestimate, the amounts of their recent purchases.